This description relates to design, structure and fabrication of infrared, thermal imaging and sensing devices, also referred to as microbolometers, based on Micro-Electro-Mechanical Systems (MEMS) structures integrated with Complementary Metal Oxide Semiconductor (CMOS) circuits.
MEMS microbolometers are wavelength-independent detectors that sense incident electromagnetic radiation by the temperature increase caused by the radiation's absorption in a sensing element. The sensing element includes a temperature-sensing material whose resistivity is dependent on temperature. The temperature (or rather temperature change) of the element then can be read-out by measuring the resistance of sensing element using associated circuitry. Detectors can be single, or arrayed in a focal plane array (FPA) to form an image
Microbolometers are typically optimized to detect infrared wavelengths in the 2-14 μm region where traditional photonic sensors are insensitive (as in the case of silicon-based Charged Coupled Devices or CMOS image sensors) or expensive to fabricate (as in the case of quantum-well devices). They can be implemented in cameras that are useful in applications such as night vision, surveillance/security, medical imaging, energy audits and search and rescue. The single elements or small arrays can be used for non-contact temperature sensing and motion and gesture detection. Vacuum packaging for MEMS microbolometers is extremely important for effective device performance. For the temperature sensing material used in microbolometers, typical temperature coefficients of resistance (TCR) are approximately 2-4%/degree Kelvin. Therefore, for low illumination intensity (or small illumination differentials), changes in the detector temperature and, hence, resistance can be extremely small. Once such differentials in resistance are equivalent to noise levels, sensitivity is lost. For maximum signal to noise performance, device architecture attempts to obtain the greatest temperature rise in the detector for any unit light input; therefore, thermal isolation of the temperature-sensing structure is critical to prevent parasitic heat transfer to the environment. For this reason, a suspended-bridge MEMS structure with narrow supports is often employed to reduce thermal conduction to the substrate and surrounding environment. Furthermore, detectors are often packaged in reduced pressure environments (<10−3 mbar) to minimize convective heat transfer to the surrounding gas. Also, since the device performance is so sensitive to the package pressure, device reliability requirements usually specify that the vacuum environment has a hermetic package seal (for example, in accordance with MIL Spec 883) to ensure leak tight performance over device lifetime.
Yield loss due to variation in pixel performance across an array or wafer is a significant contributor to the ultimate cost of microbolometer devices. The response and performance of individual pixel elements is strongly influenced by process variation and non-uniformity during fabrication. For example, non-uniformities in the temperature sensing film properties across a wafer and across the individual die can cause greatly varying device performance. Since a single focal plane array may contain between 103 to 106 pixels and each pixel must operate in a relatively narrow performance window, yield loss because of excessive performance variation is a significant issue that contributes to the cost of these devices.
The above described limitations are addressed with the design, structure and fabrication of novel imaging arrays and sensors described below.